Even as the Balrog fell, it swung its whip, and the thongs lashed and curled about Gandalf's knees, dragging him to the brink of the abyss. "Fly, you fools!" he cried, and was gone.

Lash of the Balrog

As an additional cost to cast this spell, sacrifice a creature or pay {4}.
Destroy target creature.

The Lord of the Rings: Tales of Middle-earth

Players can respond to Lash of the Balrog only after it's been cast and all its costs have been paid. No one can try to destroy the creature you sacrificed to stop you from casting this spell or to make it cost more mana.

You must sacrifice exactly one creature or pay an extra {4} to cast this spell; you can't cast it without paying one of those costs, and you can't pay either multiple times.